"I don't know how to reply to his text."
Much like English sentences, which can sometimes include 5 W's or 1 H, Chinese sentences can contain words that suggest a question or uncertainty. As long as the listener or reader understands the given sentence/question, there won't be a problem deciphering it.
The sentence you suggested also has the correct literal translation. Even though 如何, 怎么样 and 怎么 are all interchangeable, they have slightly different usages and different aspects from translators.
- 如何 commonly appears in writing since it is more formal. That indicator is the machine's favorite compound vocabulary to use.
- 怎么样/怎么 is commonly used in conversations.
As with the English "I know...", the Chinese "我知道。。。" acts sort of like a frame. What follows is simply a statement of what is known. Even if it contains "WH question words," they do not function as question words, but simply the topic that is "known."
Note that the English answer includes the word "how" even though it is a statement.
How do I answer his text? = 我怎么样回复他的短信?
I don't know how to answer his text. = 我不知道怎么样回复他的短信。